In typical power take-off devices the first shaft is an engine shaft, and the second shaft a shaft of the vehicle transmission, so that, when the output shaft is connected angularly by the drive to the second shaft, the is angular speed, corresponding to the third speed, at the power take-off end is proportional to the angular speed of the shaft transmitting power to the vehicle wheels. For this reason, the third speed is commonly referred to as “ground speed”, and is advantageously used for transmitting power to the wheels of a trailer.
In a power take-off device of the above type, selection is made, as described, for example, in US 2002/0043121, by means of two sleeves: a first sleeve is coaxial with the first shaft, and is movable axially between two work settings corresponding to the first and second speed respectively, and an intermediate idle setting corresponding to a neutral position of the output shaft; and a second sleeve is coaxial with the second shaft, and is movable axially between a work setting corresponding to the third speed, and an idle setting corresponding to a further neutral position of the output shaft.
Each sleeve can only be set to the/a respective work setting when the other sleeve is idle, and both are user-operated by respective control levers located in the vehicle cab and connected to the respective sleeves by respective drives.
Though effective and widely used, the above device has several drawbacks in production and operation relating to the mechanical complexity of the device.